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Appendix VI.

The boiler is rotated to new positions by the hand wheel £, which
turns a worm shaft n, while screw m moves the whole boiler
nearer or further from the drill. This machine is also used for
turning the boiler ends. For this purpose a compound tool-rest
is fixed in the position a;, and the boiler is then rotated con-
tinuously by power, from the cone d.

Pp. 330 6° 827. The Voltex Electric-welding System
has already been briefly explained at p. 827. It remains to
describe the apparatus itself as illustrated in Fig. 946. A wooden
handle A carries the main frame B D c, the parts B and c being of
aluminium, and D an insulator of wood. The two aluminium levers
F and G hold the carbons v and w in clamps H and j, the thumb
screws a a serving to fasten the carbons, and b b the electric leads
T u from the dynamos. The lever F is pivoted at d, but is held
in the position shewn by the spring R and stop s, the latter being
slightly adjustable. The arm G can be set at any convenient
angle by the handle K, a fine adjustment being obtained, even
when current is passing, by the vulcanite wheel N, which is turned
by the thumb, and thus moves the nut L along the screw M.
When set, the arm is held rigidly by the frictional grip of a spring
washer x. The carbons usually form an angle of 45°. Being
approximately set to the proper distance apart, the arc is struck
by pressing the lever p with the thumb. This lever is pivoted on
B, and is faced with vulcanite, and the hand is further protected
by the vulcanite shield E. When p is pressed, a pull is caused on
the wire Q, and the carbon L is thereby brought in contact with
w. On release, the spring R separates the carbons, and the arc
is formed, the best length of which is found by trial, adjusting at
N. The workman's eyes are always protected by goggles of blue
glass.

P. jjo. Thermit Welding.—It had long been known *
that aluminium was useful in reducing metallic oxides, entering
into combination with the oxygen at a very high temperature, and
leaving the metal free; but it remained for Dr. Hans Gold-
schmidt to utilise the great heat of combination as a means of
welding iron or steel, and thus provide a formidable opponent